The invention relates to a method for setting the sensitivity limits of electronic yarn clearers taking into account the fineness or count of the yarn to be cleared, the fineness being measured during the clearing process, and the result of this measurement being compared with set tolerance limits.
It is known (see, for example, USTER News Bulletin No. 29/August 1981 "Das Uster-System der Garnfehlerkontrolle" (The USTER System of Yarn Fault Control, USTER being a registered trademark of Zellweger Uster AG), the disclosure of which is hereby incorporated by reference in its entirety), that an essential precondition for accurate and reproducible yarn clearing is the establishment of a basic setting for the yarn batch to be cleared. As described in the USTER News Bulletin, yarn clearing ensures that yarn quality is suitable for subsequent processing by deleting and eliminating yarn faults. For example, typical yarn faults include deviations from a normal yarn cross-section which result in the occurrence of, for example, short thick places (e.g., a fault length of yarn less than 8 cm having a cross-section approximately twice that of the yarn), long thick places (e.g., a fault length greater than 40 cm having a cross-section approximately twice that of yarn), or thin places (e.g., having a variable fault length and a cross-section approximately 30 to 70% that of the yarn).
Although these yarn faults seldom occur, they can significantly affect the appearance of a fabric woven or processed from the yarn and/or negatively influence subsequent processing of the yarn. However, the number of faults which are actually eliminated over a given length of yarn is often limited to avoid subsequent processing difficulties. For example, as described in the aforementioned 1981 USTER News Bulletin, thin place faults thinner than 60% of normal yarn cross-section can be extracted when they are longer than approximately 40 cm, with faults shorter than 40 cm in length only being extracted if the cross-section is proportionately less than 60% of normal cross-section. Extraction of a maximum of 1-2 thin places per 100,000 cm of yarn is, for example, preferred for cotton spun yarns. Different settings can be provided for other faults, such as thick place faults and for different materials. Thus, the most important value which must be taken into account in establishing a basic setting for a yarn batch being cleared is the yarn count or fineness, which is used to fix the response limits (=tolerance values) which if exceeded give rise to a clearer step being triggered.
Since yarns of different fineness are normally spun in spinning mills, mistakes can occur, in particular in cases where the yarns differ only slightly in fineness from one another. However, even slight differences in the fineness lead to visible stripes in the woven or knitted fabric, making the product unusable. Yarn monitoring units, which measure the yarn fineness and trigger off an alarm or stop the production if the set tolerance limit is exceeded are available in order avoid this risk of confusion. However, every yarn naturally exhibits a certain unevenness, and since certain fineness deviations cannot be entirely avoided in the normal production process, difficulties are encountered in the setting of the response limits. For example, if the tolerance limits are set too narrow, false alarms frequently occur. On the other hand, if the tolerance limits are set too wide, not all faults which may be detected and eliminated without exceeding the number of correctable faults per unit length of yarn can be detected.